Evidence of dietary differentiation among late Paleocene-early Eocene plesiadapids (Mammalia, primates)

Plesiadapis cookei is an extinct relative of extant euarchontans (primates, dermopterans; scandentians), which lived in North America during the late Paleocene. P. cookei body mass has been estimated to be approximately 2.2 kg, making it large compared with other species of its genus from North America, but similar to some from Europe. In particular, size as well as dental form similarities to P. russelli have been noted. However, it is thought that P. russelli evolved from P. tricuspidens, and into Platychoerops daubrei. Dental similarities among P. cookei, P. russelli, and P. daubrei have been hypothesized to reflect a more folivorous diet than utilized by P. tricuspidens. Here we test the hypothesis that P. cookei is more dietarily specialized than P. tricuspidens by quantifying functionally significant aspects of molar, premolar, and incisor forms. Casts of M(2)s and P(4)s of P. tricuspidens, P. cookei, and P. daubrei were microCT-scanned. We measured the relief index and/or the complexity from surface reconstructions of scans. Results show that P. cookei has higher M(2) relief and complexity than P. tricuspidens; P. daubrei exhibits the highest relief and complexity. Similarly, P. cookei has a more complex P(4) than P. tricuspidens, whereas that of P. daubrei exhibits the highest complexity. Finally, the I(1) of P. cookei resembles more the incisor of P. daubrei than that of P. tricuspidens. Because high relief and complexity of dentitions are related to fibrous plant diets in living mammals, these findings support the hypothesis that previously identified similarities among P. cookei, P. russelli and P. daubrei reflect a folivorous diet.